>>23446778
-both in solar orbits and planetary orbits, and it wouldn't be too hard to drill a hole in the side to insert your cylinder if they want something defensible. Asteroids range from from less than 10 meters to dwarf planets like Ceres, which is nearly 1000 km in diameter. Island 3 cylinders are usually imagined around 32 km long, and around 8 km in diameter. IRL, the asteroid belt IS heavily disconnected but in Gundam the materials for the colonies are all extracted from them just fine without emptying the whole place and Minovski physics provides all the nuclear fusion energy everyone needs. Moreover, Gundam is a 1979 product while models for estimating the size of the asteroid belt were still made into the 90s and onward. he exact distribution and total mass of asteroids within that belt were still being explored and refined

Regardless, you could pack a ton of asteroids together and use them for shielding. In near vacuum a nuke produces almost no shockwave and all radiation, and radiation level falls off very hard in proportion to the distance between your armour and the detonation point. You will have to detonates very close to the shell for the radiation to ablate enough shell materials to produce a big enough shockwave to damage the cylinder structurally. A ton of packed asteroids together could block that easy. I suppose you'd be afraid of asteroids pushed towards your colony like shrapnel after tanking the nuke's kinetic energy that way but it's not like enemies designed a giant barrel to fire asteroids at you with nukes as a gun primer and powder.